Process for producing structurally resistant paper

ABSTRACT

Process for the production of structurally strong papers (or paper-like materials) which have a terminable strength by treatment of paper in the pulp or of finished paper in the surface with 0.0001 to 50% by weight, preferably 0.01-25% by weight (based on the pulp), of a mixture of isocyanates which contain on average at least two isocyanate groups and of which at least 20% by weight (preferably at least 50%) are those in which the NCO groups are bonded via an organic radical which contains at least one ester and/or amide group in the main chain.

This application is a 371 of PCT/EP96/01670, filed Apr. 22, 1996.

BACKGROUND OF THE INVENTION

Customary products available on the market for increasing the wet anddry strength of paper are either condensates of epichlorohydrin andpolyamines and comprise chlorine bonded organically and in salt form, orpolycondensates of formaldehyde which can release formaldehyde againunder the influence of heat and action of moisture. Both classes ofproduct therefore present environmental problems since they pollute thewaste water with chlorine or formaldehyde.

The use of products containing isocyanate groups for increasing the wetor dry strength of paper is known from EP-A 564 912.

EP-A 582 166 describes the use of polyisocyanates which have beenrendered cationic, DE-A 4 319 571 describes polyisocyanates which havebeen rendered cationic and optionally hydrophilic for this purpose, andGerman Patent Applications P 4 418 836.6 and P 4 419 572.9 describe theuse of such products in multi-component mixtures.

The content of organically bonded halogen (OX content) of the productsdescribed in the Applications cited is extremely low; consequently, theAOX pollution (AOX=adsorbable organically bonded halogen) of the wastewater of paper mills from this content is very low.

When such products are used industrially, the strengthening of the paperis very high and is also stable under hydrolytic conditions. Recyclingof these papers by digestion in dilute sodium hydroxide solution oraqueous ammonia is therefore achieved only with difficulty or not atall.

There was therefore the need for strengthening agents which impart tothe paper--without surrendering the advantages of isocyanates--a"terminable" strengthening; i.e. a strengthening which can be cancelledout during breakdown or deinking.

It has now been found that water-emulsifiable compounds which containisocyanate groups and have on average at least 2 isocyanate groupsbridged via ester and/or amide groups per mol are outstandingly suitableas dry and wet strength agents with terminable strengthening for paper,it being possible for these compounds to be employed before sheetformation (use in the pulp), i.e. as an additive to the fibroussubstance suspension, or in the surface, i.e. as an application to asheet of paper which has already been formed.

It has furthermore been found that strengthened papers which can bedigested again to a desired extent can be produced if isocyanatemixtures in which at least 20% by weight of the isocyanates containisocyanate groups bonded via ester and/or amide groups are employed forthe strengthening.

SUMMARY OF THE INVENTION

The invention relates to a process for the production of structurallystrong papers (or paper-like materials) which have a terminable strengthby treatment of paper in the pulp or of finished paper in the surfacewith 0.0001 to 50% by weight, preferably 0.01-25% by weight (based onthe pulp), of a mixture of isocyanates which contain on average at leasttwo isocyanate groups and of which at least 20% by weight (preferably atleast 50%) are those in which the NCO groups are bonded via an organicradical which contains at least one ester and/or amide group in the mainchain.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Ester groups include carbonate and allophanate groups, but not theurethane group itself. In addition to the polyisocyanates containingester and/or amide groups, customary polyisocyanates known forstrengthening in paper or else customary wet strength agents orretention agents can be present.

The invention also relates to the use of such isocyanate mixtures of thetype defined above which contain emulsifiers for better emulsificationin water, or in which the emulsifiability has been brought about byreaction of some (5-50%, preferably 8-30%) of the NCO groups withcompounds which are capable of salt formation (for exampledimethylolpropionic acid or N,N-dimethylethanolamine; (cf. DE-OS 4 319571 or P 4418836.6)) and/or hydrophilic, preferably monofuctionalpolyethers (according to DE-OS 4 211 480).

The isocyanates containing ester and/or amide groups can be prepared byreaction of isocyanates with compounds containing OH groups and havingester and/or amide groups by known processes.

Suitable isocyanates are diisocyanates, such as 1,4-diisocyanatobutane,1,6-diisocyanatohexane, 1,5-diisocyanato-2,2-dimethylpentane, 2,2,4- or2,4,4-trimethyl- 1,6-diisocyanatohexane, 1,10-diisocyanatodecane, 1,3-and 1,4-diisocyanatocyclohexane, 1-isocyanato-3 ,3 ,5-trimethyl-5-isocyanatomethyl-cyclohexane (isophorone diisocyanate),4,4'-diisocyanatodicyclohexylmethane, and mixtures thereof Aromaticisocyanates, for example tolylene diisocyanates and4,4'-diisocyanato-di-phenylmethane can in principle also be employed;however, because these are of lower fastness to light and have too higha reactivity with respect to water, the aliphatic isocyanates arepreferred. Polyisocyanates which have been obtained by modification ofthe abovementioned diisocyanates or mixtures thereof by known processesand which contain uretdione, urethane, isocyanurate, biuret orallophanate groups, can also be additionally employed as a proportion ofthe isocyanates.

Compounds containing OH groups and having ester and/or amide groupswhich are suitable are products which have on average at least 2 OHgroups and contain on average at least one ester and/or amide group.

Suitable compounds are, for example, short-chain polyesters such as areformed by esterification of diols and/or triols with dicarboxylic acidsor dicarboxylic acid anhydrides, or by transesterification withdicarboxylic -acid esters of short-chain monofunctional alcohols andremoval of these alcohols by distillation.

Acid components which may be mentioned are: dimethyl, diethyl ordiphenyl carbonate, ethylene glycol carbonate, propylene glycolcarbonate, diesters of oxalic acid and malonic acid, succinic acid,glutaric acid and maleic acid and their corresponding anhydrides, andadipic, sebacic, (also hydrogenated) phthalic and hydroxymono- or-dicarboxylic acids (if appropriate in the form of inner esters(lactones)), such as glycolic, tartaric, lactic, citric, hydroxycaproicand hydroxybutyric or ricinoleic acid.

Suitable diols are, for example, the industrially available ethane-,propane-, (1,2-or 1,3-), isomeric butane-, pentane-, hexane-diols andthe like and furthermore oligo- or polymers of ethylene glycol andpropylene glycol containing ether groups. Cycloaliphatic or aromaticdiols may be mentioned--but are not preferred because of the highviscosity of the esters. Suitable triols are, for example, glycerol andtrimethylolpropane.

Polyesters are obtained by condensation of the acids and/or their estersof monofunctional alcohols and/or of the anhydrides of the acids withthe di- or triols listed by known processes. A narrow molecular weightdistribution and therefore a low viscosity can be produced and a lowcontent of components which do not carry an ester can be achieved by useof the OH compounds in excess and subsequent extraction with water or bymolecular distillation. Ring-opening transesterification of lactones(for example butyro-, valero- or caprolactone) is likewise particularlysuitable. If appropriate, this transesterification can be coupled withthe abovementioned measures.

Particularly suitable compounds containing OH groups are obtained byreaction of a di- or hydroxy-carboxylic acid with an alkylene oxide.Defined low molecular weight ester-diols are formed in a simple mannerby this procedure.

OH compounds containing amide groups can be prepared from the acidsmentioned or esters thereof (lactones) by reaction with hydroxyamineswhich contain a secondary amino group, such as, for example, adducts ofethylene oxide or propylene oxide on a monoalkylamine (methyl-,ethylamine and the like).

This reaction is particularly suitable because, due to the selectivityof the amino groups, it leads to predominantly defined compounds. Theaverage molecular weights of the OH compounds according to the inventionare 148-2000, preferably 148-1000, particularly preferably 148-500.

To prepare the isocyanates to be employed according to the invention,compounds containing NCO are employed with the component containing OHin an NCO/OH ratio of 1.3-20, preferably 1.5-10.

At NCO/OH ratios of >1.5, a content of unreacted isocyanate--whichdepends on the structure of the isocyanate--remains. For industrialhygiene reasons, these free isocyanates should be removed, for example,by thin film distillation. (In this case, a high NCO/OH ratio is evenpreferred, because viscosity-increasing chain-lengthening reactions arethen largely suppressed.)

Esters of the hypothetical allophanic acid (so-called allophanates),which can be formed by reaction of a urethane group with an isocyanategroup, also fall within the scope of the invention. If the reaction ofthe isocyanates with the compounds containing hydroxyl groups is carriedout at ≧150° C. or in the presence of specific catalysts (such as, forexample: HCl gas or organotin compounds), the urethane groups areconverted more or less completely into allophanate groups, depending onthe reaction time. This measure offers the advantage of obtainingproducts of high isocyanate content, high functionality and lowviscosity, which is of advantage for the envisaged use.

To improve the water-dispersibility or -solubility, the isocyanates canbe provided with ionic groups, for example in accordance with DE-OS 4226 110; or DE-OS 431 957, and/or with hydrophilizing polyether chainsanalogously to DE-OS 4 211 480. Polyether addition and allophanation togive products which can be employed according to the invention can alsobe carried out in one step.

The hydrophilizing by superficial reaction of the polyisocyanates withhydrophilizing components is preferred to mixing with externalemulsifiers, which is also possible per se, because no concentration ofan emulsifier in the backwater occurs by this procedure.

The absorption of the optionally hydrophilized isocyanates ontocellulose in the pulp can be accelerated by introduction of tertiaryamino groups. For this, a low content (<30 mol %, preferably <15 mol %)of the isocyanate groups is reacted with an N,N-di-alkyl-alkanolamine(for example in accordance with U.S. Pat. No. 5,332,824).

The water-dispersible polyisocyanates are employed in the processaccording to the invention in amounts of 0.005 to 50% by weight,preferably 0.1 to 10% by weight, based on the pulp, and they arepreferably employed in the paper pulp, that is to say they are addeddirectly to the wood fibre and/or cellulose fibre dispersion. Before thepolyisocyanates are employed, especially if these are added to the paperpulp, they can be predispersed with 1-4 times the amount of water, basedon the polyisocyanate. This renders processing times of up to 24 hourspossible. If water-dispersible aromatic polyisocyanates are used, theprocessing times are shorter, because of the higher reactivity withrespect to water, and are in general less than 8 hours.

The water-dispersible polyisocyanates to be used according to theinvention can be employed at the processing temperatures customary inthe paper industry. Different processing times for the products to beused according to the invention can result here, depending on thetemperature. Thus, for example, there are relatively long processingtimes with the polyisocyanate from Example A) of DE-OS 4 211 480 (pH7±1) at 23°-25° C. because ≧60% of the isocyanate groups are stillpresent at this temperature after 5 hours. At a temperature of 35° C.,with polyisocyanate A) ≧50% of the isocyanate groups are still presentafter 5 hours, and at 50° C. ≧33% of the isocyanate groups are stillpresent after 3 hours. The processing time at a certain temperature canthus be influenced by the choice of a suitable polyisocyanate to be usedaccording to the invention.

The water-dispersible polyisocyanates used according to the inventionare suitable both for surface treatment and for pulp treatment. Theproducts to be used according to the invention can also be employed inthe size press and the like. In this manner, it is possible to producewaterproof papers which are resistant to oil and petrol. These productsaccording to the invention are also outstandingly suitable for laminatedpapers, because they cause no overloading and therefore no adverseinfluence on pigment retention, and because their strengthening actioncan be cancelled out in a simple manner. In this property, they differfrom existing systems for increasing wet strength which are currentlyused for laminates/decorative papers.

The pH of the cellulose pulp or of the paper should preferably bebetween 5 and 8.5, in particular in the neutral range, on addition ofthe products to be used according to the invention. pH values below 3 orabove 10 should be avoided.

The products to be used according to the invention allow the wetstrength of paper to be increased without pollution of the waste waterwith organic halogen compounds (determined as the AOX value inaccordance with DIN 38 409 Part 14). It is also possible to achieve animprovement in the wet tear strength directly in the machine, even undermild drying conditions. That is to say that it is not necessary--asusual--to accept considerable condensation or maturing times of theproducts. The products are furthermore distinguished in that they do notinhibit the activity of normal whiteners customary in papermaking.

If appropriate, the products can also be employed together withcustomary cationic fixing agents and retention agents or conventionalagents for increasing the wet strength.

In particular, the AOX pollution of the waste water caused by theseconventional wet-strength agents can be reduced in this way.Furthermore, the wet-strength action is usually increasedsynergistically in this manner, and the retention of pigments, fillersand the like is improved.

The water-dispersible polyisocyanates to be used according to theinvention can be employed as a mixture with 0.005 to 50 parts by weight,preferably 0.1 to 10 parts by weight, of a retention agent, the amountof retention agent being based on the paper pulp.

The water-dispersible polyisocyanates to be used according to theinvention can be employed as a mixture with 1 to 400 parts by weight,preferably 10 to 100 parts by weight, of apolyamidoamine-epichlorohydrin resin according to DE-AS 1 177 824, basedon the polyisocyanate.

To test the terminable strengthening, the papers which have been treatedand subjected to after-condensation can be broken down again in a pulperor subjected to deinking conditions (H. L. Baumgarten et al."Deinking-Entwicklungsstand einer Schlusseltechnologie fur dieAltpapierverwertung" Deinkng-Development Status of a Key Technology forWaste Paper Utilization!, Das Papier 42 (1988) V166-V177 and W. Berndt"Die Chemikalien der Deinking-Prozesse" The Chemicals of the DeinkingProcesses! Wochenblatt 15 (1982) 533-541). A sheet of paper is thenproduced again and it is investigated whether this sheet is speck-free(i.e. free from poorly digested, too easily produced fibreagglomerates). It is easier to test a sheet for wet strength in thecustomary manner, and then to expose an identical sheet to aconcentrated ammonia solution or a 1N sodium hydroxide solution at 50°C. for 24 hours, to neutralize it, if appropriate, and to dry it and todetermine the wet strength once again.

Papers which can easily be broken down are obtained if the wet strengthof the paper treated with alkali is only 75%, preferably only 50%, andparticularly preferably <30%, of the initial wet strength.

Preparation examples for the isocyanates according to the invention

EXAMPLE A

1164 g (6 mol) of tetraethylene glycol, 184 g (2 mol) of glycerol and792 g (6 mol) of dimethyl malonate were heated at 140° C. under nitrogenwith the addition of 150 mg of dibutyltin dilaurate as atransesterification catalyst. The distilling off of methanol whichstarts was maintained by gradually increasing the temperature to 180° C.Thereafter, the condensation was brought to completion in the course of4 hours at the same temperature and under decreasing pressure (finally15 mbar). A low-viscosity, on average trifunctional oligoester of OHnumber 192 was obtained.

877 g (3 mol of OH) of the oligoester were stirred with 1764 g (10.5mol) of hexamethylene diisocyanate at 80° C. under nitrogen for 4 hours.Thereafter, the isocyanate content had fallen to a calculated 28.6%. Thecrude product was freed from monomeric diisocyanate by distillationtwice on a molecular evaporator (jacket temperature 140° C. pressure 0.5mbar). 1350 g of a yellowish oil having an NCO content of 8.3% and aviscosity of 9080 mPa.s at 25° C. were obtained.

EXAMPLE B

2565 g (22.5 mol) of ε-caprolactone were stirred with 1005 g (7.5 mol)of trimethylolpropane under nitrogen for 8 hours at 200° C. Thereafter,only traces of the monomeric lactone were still detectable. Thelow-viscosity oligoester had an OH number of 347.

485 g (3 mol of OH) of the oligoester were stirred with 1260 g (7.5 mol)of hexamethylene diisocyanate at 65° C. for 1 hour and at 80° C. for 3hours, and thereafter the calculated NCO content (28.9%) had beenreached.

After molecular distillation twice, a viscous oil of viscosity 21 250mPa.s having an NCO content of 9.9% was obtained.

75 parts of the isocyanate were diluted with 25 parts of methoxypropylacetate and reacted with 25 parts of a polyethylene glycol monomethylether (molecular weight=350) at 80° C. in the course of 4 hours forbetter emulsifiability in water. The resulting solution had an NCOcontent of 3%.

EXAMPLE C

A low-viscosity oligoester of OH number 458 was prepared analogously toExample B from s-caprolactone and trimethylolpropane in a molar ratio of2:1.

After reaction with hexamethylene diisocyanate in excess (NCO:OH=5) andmolecular distillation, a prepolymer of low monomer content having aviscosity of 23,400 mPa.s at 25° C. and an NCO content of 11.33% wasobtained.

Modification with 25% of polyethylene glycol monomethyl ether (molecularweight=350) in methoxypropyl acetate gave a water-emulsifiable liquidhaving an NCO content of 3.45%.

EXAMPLE D

2226 g (21 mol) of diethylene glycol were reacted with 1197 g (10.5 mol)of ε-caprolactone under nitrogen at 200° C. for 5 hours. The crudeproduct was subjected to molecular distillation twice under 0.3 mbar,the jacket temperature being 100° C. in the first pass and 120° C. inthe second.

An ester-diol of OH number 421.5 which was largely free from diethyleneglycol was obtained.

266 g of the ester-diol (2 mol of OH) were mixed with 1718 g (10.22 mol)of hexamethylene diisocyanate, and 6 g of gaseous HCl (0.16 mol) werepassed into the mixture. After the mixture had been stirred at 110° C.for 8 hours, the NCO content of the mixture had fallen to 34.67%, whichcorresponds to complete reaction of the urethane groups initially formedwith further isocyanate to give allophanate groups.

After molecular distillation twice at 190° C. and 0.5 mbar, apolyisocyanate of low monomer content which had a viscosity of 2970mPa.s at 25° C. and an NCO content of 15.7% was obtained.

For better emulsifiability in water, the product was modified withpolyethylene glycol monomethyl ether as in Examples B and C. it beingpossible to dispense with an addition solvent (NCO:9.35).

EXAMPLE E

133 g (0.5 mol) of the ester-diol from Example D were mixed with 255 g(0.5 mol) of polyethylene glycol monomethyl ether (molecular weight=550)and 1250 g (7.44 mol) of hexamethylene diisocyanate, and 4 g (0.11 mol)of gaseous HCl were added. After 6 hours at 110° C. completeallophanation of the urethane groups was achieved (NCO:30.4%).

After molecular distillation twice at 140° C. and 0.4 mbar, alow-viscosity polyisocyanate having an NCO content of 12.07% and aviscosity of 650 mPa.s at 25° C. was obtained. The product was readilyemulsifiable in water, due to the content of polyethylene glycolpolyether, and required no further modification for use.

EXAMPLE F

Analogously to Example E, 200 g (0.75 mol) of the same ester-diol and128 g (0.25 mol) of polyethylene glycol monomethyl ether (molecularweight=550) were reacted with 1470 g (8.75 mol) of hexamethylenediisocyanate in the presence of 4 g of HCl at 110° C. in the course of 6hours, and the resulting crude product (NCO:32.5%) was freed frommonomeric diisocyanate by molecular distillation.

A water-emulsifiable polyisocyanate having an NCO content of 13.84% anda viscosity of 1340 mPa.s at 25° C. was obtained.

EXAMPLE G

1 g of N,N'-dimethylethanolamine and 0.05 g of 2-chloropropionic acidwere added to 100 g of the isocyanate of Example B diluted to 75%strength with methoxypropyl acetate, and the mixture was kept at 80° C.for 24 hours. A product which contained 1.01% of NCO was formed.

EXAMPLE H

1 g of N,N-di-ethylethanolamine and 0.1 g of 2-chloropropionic acid wereadded to 100 g of an isocyanate of Example C diluted to 75% strength(with methoxypropyl acetate), and the mixture was stirred at 80° C. for10 hours. A prepolymer with 2.22% of NCO was formed.

EXAMPLE J

60 parts of the isocyanate from Example C were dissolved in 20 parts ofmethoxypropyl acetate and reacted with 20 parts of a polyethylene glycolmonomethyl ether (of molecular weight 350) and 1 part ofdiisopropyl-ethanolamine. An isocyanate having 4.0% of NCO and aviscosity of 1520 mPa.s at 25° C. was formed.

EXAMPLE K

340 g of isocyanate prepared according to Example F were reacted with 17g of polyethylene glycol monoethyl ether (molecular weight 350) and 4.5g of N,N-dimethylethanolamine at 60° C. An isocyanate which can beemulsified in water even better than isocyanate F and has an NCO contentof 11.6% and a viscosity of 1750 mPa.s at 25° C. was formed.

USE EXAMPLES Example 1

This example shows the activity in the paper pulp of the polyisocyanatesto be employed according to the invention.

A mixture of 50% each of bleached birch sulphate cellulose and pinesulphate cellulose was beaten in a beater at a pulp consistency of 2.5%to a Schopper-Riegler degree of freeness of 30°. 100 g of this mixturewere introduced into a glass beaker and diluted to 1000 ml with water.

The amounts of polyisocyanate A (based on the fibre substance) stated inthe following table were introduced into the glass beaker as an aqueousdispersion. This dispersion was prepared as follows:

10 g of polyisocyanate A were emulsified, with the aid of

1 g of Emulgator VA (Bayer AG; an emulsifier based on alkoxylatedstearylurethane), in

89 g of water.

5%, 10% and 20% (based on the cellulose fibres) of this emulsion (basedon the dispersed isocyanate) were added.

After a stirring time of 3 minutes, sheets of paper having a weight perunit area of about 80 g/m² were formed with the contents of the glassbeakers on a sheet-forming machine (Rapid-Kothen device). The sheets ofpaper were dried at 85° C. in vacuo under 30 mbar for 8 minutes and,after 10 minutes, after-heated in a drying cabinet at 110° C.

After conditioning, 5 test strips 1.5 cm wide were cut out of each sheetof paper and immersed in distilled water for 5 minutes. Thereafter, thewet strips were immediately tested for their wet breaking load in atensile tester. A wet-strength agent with isocyanate groups, preparedaccording to Example 1 of EP-A 564 912, was furthermore also tested as astandard.

The paper with polyisocyanate A according to the invention (drying at85° C. 8 minutes) had the following wet strengths:

    ______________________________________                                                 5%          10%    20%                                               ______________________________________                                        Isocyanate A                                                                             4.6           7.6    8                                             Isocyanate of                                                                            9.3           9.7    --                                            Example 1                                                                     EP-A 564 912                                                                  Isocyanate of                                                                            9.5           12.7   --                                            Example 1                                                                     EP-A 564 912 +                                                                emulsifier                                                                    ______________________________________                                    

Example 2

The experiments of Example 1 were repeated on a pulp of 80% pinesulphate and 20% birch sulphate (degree of freeness 35%) with adispersion for which 3.0% of isocyanate A was dispersed in 91% of tapwater with 6.0% of Emulgator VA.

After production of the, sheet of paper, condensation was carried out at110° C. for 10 minutes. The resulting sheet had the following wetstrengths:

1% of A: 1.4 N

5% of A: 2.2 N

10% of A: 7.1 N

Under breaking down conditions, the paper was broken down again at 50°C. and a pH of 11 with 2.5% of potassium persulphate within a stirringtime of 40 minutes.

A new sheet produced therefrom was free from specks.

Example 3

These examples show the action of the products according to theinvention in coatings.

75 g/m² coated base paper comprising 70% birch sulphate and 30% pinesulphate (degree of freeness 35%) and 20% chalk (as well as 0.5% of aretention agent (RETAMOL C 01 from Bayer AG) were treated in the sizepress with a dispersion which comprised the following products. Thepaper in each case absorbed the amount of isocyanate indicated. Aftermeasurement of the wet breaking load, the papers--in order to determinetheir ability to be broken down again--were immersed in 1N NaOH or NH₃solution at 50° C. for 1 hour, heated and cooled, and, after standing atroom temperature for 16 hours, the wet breaking load was determinedagain. If the wet breaking load did not differ substantially from thatof the untreated paper, then the paper also cannot be digested again.

    __________________________________________________________________________    Example I                                                                     EP-A 564 912                                                                  Comparison                                                                            Isocyanate A                                                                         Isocyanate B                                                                         Isocyanate C                                                                         Isocyanate D                                                                        Isocyanate E                                                                        Isocyanate F                         0.5%                                                                              1%  0.5%                                                                              1% 0.5%                                                                             1%  0.5%                                                                             1%  0.5%  0.5%  0.5%                                                                             1%                                __________________________________________________________________________    12  12.7                                                                              4.6 9.3                                                                              3.8                                                                              12.3                                                                              7.9                                                                              12.5                                                                              13.3  13.3  10.5                                                                             13.4                                                                              Wet breaking load without                                                     alkili                                                                        treatment                     10  12.7                                                                              0.5 0.6                                                                              0.5                                                                              1.5 4.4                                                                              7.7 9.6   9.8   7.6                                                                              9.8 Wet breaking load after                                                       NaOH treatmeat (pH 14)        11  13.5                                                                              2.5 7.9                                                                              2.1                                                                              10.9                                                                              6.5                                                                              11.4                                                                              11.7  12.1  9.4                                                                              12.7                                                                              Wet breaking load after                                                       ammonia                                                                       treatment (pH                 __________________________________________________________________________                                                    12)                       

Example 4

Screenings were treated in the pulp with in each case the stated amountof freshly emulsified isocyanate analogously to Example 1 and the papersthus produced were tested. The wet breaking load was determined in eachcase after condensation (30 minutes at 110° C.) and after a hydrolysistreatment.

The following results were obtained:

    __________________________________________________________________________    Example 4.1                                                                              4.2                                                                              4.3                                                                              4.4                                                                              4.5                                                                              4.6                                                                              4.7                                                                             4.8 4.9                                                                              4.10                                                                             4.11                                    __________________________________________________________________________    Active                                                                        substance in %                                                                Example 1                                                                             1.5                                                                              0.5                                                                              -- -- -- -- --                                                                              1.5                                                                              0.5 -- --                                      EPA 564 912                                                                   Example 1                                                                             0  1.5                                                                              2  1.5                                                                              0.5                                                                              -- --                                                                              -- --  1.5                                                                              0.5                                     P 4 319 571.1                                                                 Isocyanate C                                                                          0.5                                                                              -- -- 0.5                                                                              1.5                                                                              -- --                                                                              -- --  -- --                                      Isocyanate E                                                                          -- -- -- -- -- 2.0                                                                              --                                                                              0.5                                                                              --  0.5                                                                              --                                      Isocyanate F                                                                          -- -- -- -- -- -- 2.0                                                                             -- 1.5 -- 1.5                                     Wet breaking                                                                          12.6                                                                             9.4                                                                              18.4                                                                             18.2                                                                             12.6                                                                             11.1                                                                             9.5                                                                             12.8                                                                             11.7                                                                              18.2                                                                             14.7                                    load  N!                                                                      Wet breaking                                                                  load  N! after 1                                                              N NaOH                                                                        treatment                                                                     20 hours 50° C.                                                                5.0                                                                              2.8                                                                              14.4                                                                             9.8                                                                              5.0                                                                              1.9                                                                              1.9                                                                             6.2                                                                              3.1 11.4                                                                             7.7                                     63 hours 50° C.                                                                3.6                                                                              2.1                                                                              13.6                                                                             8.5                                                                              3.1                                                                              1.4                                                                              1.2                                                                             3.8                                                                              2.2 5.3                                                                              2.8                                     __________________________________________________________________________

Example 5

0.2% of RETAMINOL K (cationic retention agent from Bayer AG) and theamounts of isocyanate stated in the table, in emulsified form, wereadded to a cellulose mixture comprising 30% pine sulphate and 70%eucalyptus, to which 40% titanium dioxide had been added, sheets wereformed with this mixture on a Rapid-Kothen and the ash and wet strengthwere determined. To determine the terminable wet strength, samples weretreated with 1N NaOH at 50° C. for 20 hours and the wet strength wasdetermined again. If it was significantly lower than in the sample nottreated with NaOH, the resulting sheets can easily be broken down again.

    __________________________________________________________________________    Example    5.1                                                                              5.2                                                                              5.3                                                                              5.4                                                                              5.5                                                                              5.6                                                                              5.7                                                                              5.8                                                                              5.9                                                                              5.10                                    __________________________________________________________________________    RETAMINOL K (%)                                                                          0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                     NADAVIN DHN                                                                              2.25                                                                             15 -- -- -- 8  -- -- --                                         comparison (%)                                                                Isocyanate E (%)                                                                         -- -- 2.8                                                                              15 -- --                                                  Isocyanate F (%)                                                                         -- -- -- -- 2.8                                                                              15 -- -- 8                                          Example 1 EP-A 564 912                                                                   -- -- -- -- -- -- 2.8                                                                              15 -- 8                                       comparison (%)                                                                Example 1 P 4 319 571.7                                                                  -- -- -- -- -- -- -- -- 2.8                                                                              15                                      comparison (%)                                                                Ash (%)    21 12.1                                                                             16.6                                                                             16.2                                                                             15.4                                                                             14.2                                                                             18.5                                                                             19.9                                                                             17.1                                                                             21.5                                    Wet breaking load  N!                                                                    16.6                                                                             19.2                                                                             7.7                                                                              9.8                                                                              6.8                                                                              11.2                                                                             6.7                                                                              7.4                                                                              13.8                                                                             12.3                                    Wet breaking load after                                                                  -- 12.1                                                                             -- 3.8                                                                              -- 4.0                                                                              -- 7.6                                                                              -- 11                                      hydrolysis (20 hours 1 N                                                      NaOH at 50° C.)                                                        __________________________________________________________________________     All the % data are based on the active substance                              Examples 5.1 and 5.2 clearly show how, with conventional wetstrength          agents, TiO.sub.2 retention decreases due to overloading. At a lower          concentration of the conventional wetstrength agent DHN, the ash is high;     at 21%, and at the higher concentration of 15% it is low, at 12.1%. With      the wetstrength agents according to the invention, the ash content is         largely independent of the concentration used.                           

Example 6

2% (dry) of Comparison Example 1 from EP-A 564 912 (Experiment 6.1), andisocyanate G (Experiment 6.2) and isocyanate H (Experiment 6.3), areadded to a cellulose mixture comprising 80% pine sulphate and 20% birchsulphate.

The following wet strengths (after drying at 110° C. for 3 minutes) weremeasured the following day:

    ______________________________________                                        Experiment No. Wet breaking load                                              ______________________________________                                        6.1            5.0                                                            6.2            9.3                                                            6.3            12.7                                                           ______________________________________                                    

After storage over an ammonia atmosphere, the values dropped to:

6.1 3.5

6.2 1.0

6.3 1.2

Example 7

This example is intended to illustrate the comparability of thehydrolysis test (ageing for 8 or 10 hours with 1N NaOH or over ammoniavapour at 50° C.) with the actual results of breaking down:

An 80 g/m² paper comprising 70% birch sulphate and 30% pine sulphate(degree of freeness 35), which comprised 12.5% chalk and had a wetuptake of 85%, was treated with various wet-strength agents in the sizepress. The wet breaking load of these papers was determined. The paperswere then subjected to hydrolysis (20 hours, 50° C. in 1N NaOH), and thewet breaking load was determined again in the manner described.Identical papers were broken down again in 1N NaOH at 50° C. for 30minutes, the fibre slurry formed was neutralized and a sheet was formedagain. This sheet was then examined and evaluated for specks, which canserve as a measure of the ability to be broken down again. (Rating 1:cannot be broken down, fairly large pieces of the original paper stillto be seen, rating 5:

speck-free paper).

    __________________________________________________________________________                  Active                                                                    NCO compound                                                                            Viscosity                                                 Product   content                                                                           content (%)                                                                         (mPas)                                                                             % %  %  %  %  %  %  %                                __________________________________________________________________________    NADAVINDHN                                                                              --  15     70  0.5                                                                             1  -- -- -- -- -- --                               Example 1 12.8                                                                              80     500 --                                                                              -- 0.5                                                                              1  -- -- -- --                               EP-A 564 912                                                                  Example J 4   80    1520 --                                                                              -- -- -- 0.5                                                                              1  -- --                               Example K 11.6                                                                              100   1750 --                                                                              -- -- -- -- -- 0.5                                                                              1                                Wet breaking load (N)    8.2                                                                             11.8                                                                             12.8                                                                             14.8                                                                             10.2                                                                             15.7                                                                             15.4                                                                             17.9                             Wet breaking load        2.2                                                                             4  10.2                                                                             11.9                                                                             0.4                                                                              0.6                                                                              5.3                                                                              7.5                              20 hours, 1N NaOH,                                                            50° C.                                                                 Residual wet breaking    27                                                                              34 80 80 4  4  34 42                               load after hydrolysis                                                         in %                                                                          Ability to be broken       4     1     5     4                                down: appearance of                                                           the re-formed paper                                                           __________________________________________________________________________

We claim:
 1. Process for the production of structurally strong papershaving a terminable strength comprising, adding to a papermaking pulp orto the surface of a finished paper 0.0001 to 50% by weight (based on thepulp), of a mixture of isocyanates which contain on average at least twoisocyanate groups and of which at least 20% by weight are those in whichthe NCO groups are bonded via an organic radical which contains at leastone ester and/or amide group in the main chain.
 2. Process according toclaim 1, further comprising, the addition of isocyanate size and/or wetstrength and/or retention agents.
 3. Process according to claim 1,wherein the isocyanates are hydrophilized by proportionate reaction withmonofunctional polyethylene glycol ethers and/or by cationic or anionicgroups.
 4. Process according to claim 1, wherein the isocyanates arealiphatic.
 5. Process according to claim 1, wherein the isocyanatemixture comprises an isocyanate containing ester and/or amide groups inan amount such that the finished treated paper has, after hydrolysis in1N NaOH at 50° C. for 8 hours, not more than 75% of the wet strength ofthe non-hydrolysed paper.